Research On Precision Improvement Of Portable Near-infrared Spectrometer

The near-infrared spectroscopy has been widely applied to agriculture, pharmacy and petrochemical industry because of its advantages of rapid analysis and nondestructive measurement. Restricted by the volume, power consumption and working environment, the precision of the portable near-infrared(NIR) spectrometers is lower than the precision of the laboratory instruments. So their application and development has been affected seriously. Therefore, the research of the method and experiment were provided to improve the precision of the portable NIR spectrometer PISA-S-4N, which was self-developed based on the digital lock-in amplifier technology.Aiming at the poor precision of the portable NIR spectrometer PISA-S-4N, the modules and related parameters to influence the precision were confirmed by analyzing the structure of spectrometer and transmission process of the signal. They are the stability of modulation frequency in the chopper steady speed control circuit, the stability of temperature in the sensor temperature control module, the frequency adaptability of demodulation algorithm in the digital sampling and control module. The corresponding improvement programs were also presented. The chopper steady speed control circuit was redesigned based on the proportion integration differentiation(PID) technology, the H bridge motor driver chip and the microcomputer controller. The influence of manufacture and installation errors of chopper was reduced by program controlling to increase stability of modulation frequency. The sensor temperature control module was redesigned based on the PID technology, the bridge type temperature measuring circuit, the dedicated driver chip of semiconductor refrigerating machinery and the microprocessor to increase stability of temperature control. The digital sampling and control module was redesigned based on the simplified digital lock-in amplifier with frequency adaptability. The full period demodulation algorithm was implemented in microprocessor. Comparing with the digital sampling and control module in PISA-S-4N, the circuitry was simplified, the arithmetic was simple.The experimental results shown that the improved modules and their applications in NIR spectrometer obtained higher precision than the modules in PISA-S-4N. The frequency fluctuation of chopper steady speed control module reduced from 5.67 Hz to 3.01 Hz, and the signal to noise ratio(SNR) of spectrometer was raised from 1777 to 3496. The temperature fluctuation of sensor temperature control module reduced from 0.89℃ to 0.11℃, and the SNR of spectrometer was raised to 2259. The digital sampling and control module was improved by the full period demodulation algorithm, achieved the simplify of the hardware circuit and the optimization of software algorithm, and the SNR of spectrometer was raised to 3827. The SNR was raised to 4938 when the three improved modules were integrated into the portable NIR spectrometer.